Peripheral arterial monitoring instruments

ABSTRACT

A method and apparatus are described for determining characteristics of peripheral arterial volume and compliance. A blood pressure cuff is inflated and deflated around a limb of the body and pressure measurements are taken. The volume of air removed from the cuff is determined in a quantifiable manner, such as by expelling air through an orifice of known characteristics or by means of a volume of known characteristics. The detected pressures and volume of air removed are used to compute oscillation volume, which in turn is used to display arterial capacity and compliance as a function of transmural pressure and time. Arterial capacity may be displayed in terms of arterial radius, arterial cross-sectional area, or arterial volume. Also, systolic and pulse pressures are determined using only these determined values.

This is a continuation, of application Ser. No. 08/056,103, filed May 3,1993. U.S. Pat. No. 5,417,220, and a continuation-in-part of Ser. No.823,909, filed Jan. 22, 1992, U.S. Pat. No. 5,218,968, and Ser. No.453,519, filed Dec. 20, 1998, U.S. Pat. No. 5,103,833, which is herebyincorporated by reference.

FIELD OF THE INVENTION

This invention relates to monitoring instruments which provideinformation concerning peripheral vasculature and, in particular, to theuse of such instruments to provide medical diagnostic informationconcerning arterial volume, cross-sectional area, and compliance, andespecially, diastolic and systolic pressure.

BACKGROUND OF THE INVENTION

The medical conditions of arteriosclerosis and hypertension arepotentially debilitating and often life-threatening conditions whichrequire early diagnosis and treatment. These conditions arecharacterized by changes in arterial blood flow volumes and rates andthe response of arterial tissue to changes in blood pressure. Aphysiological phenomenon which plays a part in these arterialcharacteristics is referred to herein as arterial compliance, theability of vasculature to respond to changes in these conditions. Thearterial walls of the body include collagen, giving the walls theability to expand and contract, and muscle tissue which in part controlsthis expansion and contraction. Vascular compliance includes theresponse of collagen and muscle in arterial walls to changingconditions. In addition, the condition of arteriosclerosis ischaracterized by the buildup of fatty substances along arterial walls.These substances can occlude the artery, and can impede the ability ofthe arterial walls to respond to changing conditions of blood pressure.The fatty substances characteristic of arteriosclerosis are thus afurther factor governing arterial compliance. It is thus desirable to beable to analytically understand arterial volume and compliance whendiagnosing or treating the medical conditions of hypertension andarteriosclerosis.

An understanding of a patient's arterial volume and compliance is alsobeneficial when administering anesthesia. The quantity of anestheticadministered to a patient should be just sufficient to eliminate aphysiological response by the patient during surgery. If an insufficientamount of anesthetic has been administered, the cardiovascular systemwill respond reflexively when the patient is intubated prior to surgery.This response can be detected by monitoring arterial volume andcompliance, and noting any reduction in these characteristics duringintubation. A cardiovascular response can also be detected at the timeof the first surgical incision, when an insufficient anesthetic willagain be evidenced by a reduction in arterial compliance or volume.Thus, a surgical patient would benefit from the monitoring of arterialvolume and compliance by the anesthesiologist and surgeon.

The significance of arterial volume and compliance has been recognizedin the prior art. In a sequence of patents including U.S. Pat. Nos.3,903,872; 4,565,020; 4,651,747; and 4,712,563 issued to William T.Link, methods and apparatus are described for calculating measurementsof arterial volume and compliance. Link's technique as described inthese patents involves taking and using a series of standardoscillometric blood pressure measurements. The first time derivative ofthe measured cuff pressure pulse, dP, at a time within a patient'sactual blood pressure pulse as a function of applied cuff pressure isthen calculated to inferentially determine arterial volumetric changesdV. As Link shows, this first derivative corresponds to changes inarterial volume changes dV. A curve plotted from these calculations istransformed by Link to a curve of volumetric change as a function oftransmural pressure, V/P, and this curve may in turn be differentiatedto obtain a compliance curve dV/dP.

In a second sequence of patents including U.S. Pat. Nos. 4,664,126;4,697,596; 4,699,151; and 4,699,152, Link extends this analysis to atechnique in which the peak to peak amplitude of each cuff pulse and thepatient's diastolic and systolic pressures are used to calculate aparticular patient's own volumetric and compliance curves. Again, thevolumetric and pressure information is determined inferentially fromarterial pressure pulse information. The volumetric and pressure curvesare used by Link in the determination of systolic, diastolic, and meanarterial blood pressure. The Link technique utilizes a ramp-up method ofmeasuring pressure pulses, wherein pulse data is taken during inflationof a blood pressure cuff. The currently preferred technique for takingsuch measurements, which is incrementally deflating a blood pressurecuff from a pressure level in excess of systolic pressure and takingmeasurements over a range of declining pressure steps, is described inU.S. Pat. Nos. 4,349,034 and 4,360,029, issued to Maynard Ramsey, III.

A display of information concerning arterial volume which is useful tothe anesthesiologist, surgeon or diagnostician is a curve representingarterial volume (in cc) or area (in mm²) as a function of transmuralpressure (in mm Hg). At a given point on the positive pressure side ofthis curve the volume or area may be represented by a value R, theeffective arterial radius. The slope of the curve at any given point,dV/dP, represents arterial compliance, and a plot of dV/dP as a functionof transmural pressure represents the arterial compliance curve.

In accordance with the principles of a parent of this invention, Ser.No. 453,919, now U.S. Pat. No. 5,103,833, the patient's arterial volumeand compliance is represented in this format and, in correspondencethereto, the value of R over time is calculated and displayed. Thedisplay of this data provides the anesthesiologist with informationconcerning the patient's arterial volume and compliance characteristics,and also provides information as to changes occurring in arterial volumeover time. This will enable the anesthesiologist to detect any responseof the cardiovascular system to intubation or incision during a surgicalprocedure, thereby facilitating the correct delivery of anesthetic tothe patient.

A display as described above may be further enhanced by providing thearterial compliance dV/dP at a given transmural pressure for a patientundergoing diagnosis or monitoring. The maximum value of dV/dP, referredto as peak arterial compliance, can also be ascertained from thisinformation. A further display of this information which would be of useto a clinician would be a representation of arterial capacity, R, inrelation to the radius of the limb at which the blood pressure cuff ofthe monitoring instrument is attached.

In accordance with another aspect of the aforementioned patent, avariation of this display format provides a display of the patient'sarterial volume data prior to the initiation of any surgicalintervention and, in correspondence therewith, a current display ofarterial volume data as the surgical intervention proceeds. Comparisonof the data informs the anesthesiologist of the cardiovascular systemresponse to bodily stimuli during the procedure.

A recent proposal relating to the determination of vascular complianceis known as the "Hartsafe Product Concept." This product concept isfurther described in Raines, Jaffrin and Rao, "A NoninvasivePressure-Pulse Recorder: Development and Rationale", MedicalInstrumentation, Vol. 4, Sep.-Oct. 1973, pp. 245-250. In this procedure,a pressure cuff is strapped to a patient's calf and inflated. When thecuff pressure attains a level of 70 mm Hg, a calibration step isinitiated by injecting one m1 of air into the cuff. The system measuresthe change in pressure resulting from this quantified injection andcalculates a calibration factor based upon the change. Cuff inflationcontinues and volume pulse signals are recorded until a minimal volumepulse signal or a maximum pressure value of 225 mm Hg is attained. Thesystem then commences a step deflate sequence. At individual pressuredecrement steps of 10 mm Hg the volume pulse signal is recorded. Thesequence continues until a minimal pressure level is attained, at whichtime data acquisition is complete. The system then performs "signalconditioning" using the volume pulse and cuff pressure signals at each10 mm Hg cuff pressure decrement, and the calibrate signal previouslystored. The volume-pressure curve, peak compliance, and other parametersare obtained by this "signal conditioning." The Raines/"Hartsafe"approach seems to be a more direct measurement of arterial volume thanthe Link techniques, in which arterial volume is calculated premisedupon its relationship to the arterial pressure pulses, because an actualmeasurement of system response to a known change in cuff volume is takenduring the calibration step of "Hartsafe." But the actual data which iscomputed for the volume-pressure curve appears to be similarlyinferential, however, as the single calibration volume measurement isthe only volumetric measure used in conjunction with the pulse signalsto inferentially calculate the curve.

It would be desirable to provide arterial volume and complianceinformation that is based upon direct measurements of arterial volumeand changes in arterial volume. It would further be desirable tocontinually recalibrate the system during the acquisition of suchvolumetric data, or to obviate the need for calibration entirely byobtaining highly accurate volumetric data in the first instance. Inaccordance with a further aspect of Apple, U.S. Pat. No. 5,103,833, asystem for measuring arterial cross-section and compliance is providedin which a pressure cuff on a peripheral part of the body is inflated toa pressure level which occludes arterial vessels. The cuff is thendeflated, and pressure measurements taken in correspondence withdecreasing pressure levels. Air which is expelled during deflation isremoved from the cuff through means for determining the volume of airexpelled. This means may comprise, for instance, an orifice or transfervolume of known characteristics, or a flow measurement device. At eachpoint at which a pressure determination is made, the volume of airremoved from the cuff is precisely known, or is calculated based upon animmediately obtained volumetric calibration. Thus, there is no need forthe use of a calibration factor or reliance upon a single priorcalibration step in the determination of arterial volume and complianceperformed by the system.

As air is removed from the pressure cuff in accordance with the Appleinvention, the oscillation pressure peaks and changes in cuff volume asa function of pressure are recorded over a range of cuff pressures. Fromthis information the oscillation volume is calculated. From theknowledge of oscillation volume measurements over the range of cuffpressures and conventional determination of systolic and diastolicpressure levels, the patient's arterial volume and compliance curves arereconstructed. Thus, accurate and complete information concerning bloodpressure and arterial volume, cross-section and compliance in relationto transmural pressure and/or time is provided to the physician formonitoring and diagnosis.

SUMMARY OF THE INVENTION

The methods and apparatus described by this invention disclose a meansfor performing measurements of vital signs which includes only: aninflatable cuff; inflating and deflating means connected to the cuff;means coupled to the inflating and deflating means which expels air fromthe cuff in decremental amounts, so that it may be quantified; andcomputational means which use these determined cuff pressures anddecremental volumes to determine arterial volume, arterial capacity(compliance), volume versus pressure, and, importantly, diastolic andsystolic blood pressures of the patient. Methods are also described formaking all the above determinations. Importantly, it should be realizedthat the methods and apparatus herein described do not require a priorknowledge of the patient's systolic and diastolic pressures. Thissignificant advancement allows it to be possible to construct an actual(and not proportional) pressure-volume curve, to determine parameterssuch as arterial volume, arterial diameter and arterial compliance atdifferent arterial wall pressures, and to determine better the diastolicand systolic pressures, which are ultimately more accurate than thedeterminations made using current techniques.

This invention will be better understood when read in connection withthe attached Description of the Drawings taken in conjunction with theDetailed Description of the Invention.

DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 illustrates a peripheral arterial monitoring instrument of thepresent invention attached to the thigh of the body;

FIGS. 2a and 2b illustrate two types of vascular information displays ofthe instrument of FIG. 1;

FIGS. 3a and 3b illustrate a peripheral limb of the body in relation tothe information displays of FIGS. 2aand 2b;

FIG. 4 illustrates schematically the connection of a arterial monitoringinstrument to a limb of the body;

FIG. 5 is a schematic illustration of a peripheral arterial monitoringinstrument of the present invention which uses an orifice for measureddeflation;

FIG. 6 is a graphical illustration of step deflation using an orificefor calibrated deflation;

FIG. 7 is a schematic illustration of a peripheral arterial monitoringinstrument of the present invention which uses a transfer volume formeasured deflation;

FIG. 8 is a graphical illustration of step deflation using a transfervolume for calibrated deflation;

FIG. 9 is a graphical representation of flow versus pressure across thedeflate orifice;

FIG. 10 is a graphical representation of oscillation pressure versuscuff pressure;

FIG. 11 is a graphical representation of volume reconstruction versuscuff pressure;

FIG. 12 is a graphical representation of estimated values using themethod of the present invention of volume versus wall pressure;

FIG. 13 is a graphical representation of the values of FIG. 12 afterusing optimization techniques;

FIG. 14 is a graphical representation of the estimated values of volumeparameters made using methods of the present invention; and

FIG. 15 is a graphical representation of the values of FIG. 14 afterusing optimization techniques.

DETAIL DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, a peripheral arterial monitoring instrumentconstructed in accordance with the principles of the present inventionis shown in use on the leg of a patient. The instrument includes aconventional blood pressure cuff 10 having a length l which is wrappedabout the thigh of the patient. Although the cuff 10 maybe applied toany peripheral part of the body and is most conventionally applied tothe upper arm, it is preferable to use the thigh in some procedures asthat is where buildups of occlusive substances leading toarteriosclerosis and the like generally first manifest themselves. Inother applications the upper arm or finger may be a preferred site forapplication of the cuff. The cuff 10 is connected by tubing 12 to amonitor and processor 14. The monitor and processor 14 includes a numberof controls for actuating and adjusting the instrument in theperformance of vascular measurements including blood pressuredetermination. The monitor and processor also includes a display 16where the data taken during measurements of arterial volume isdisplayed, either in numerical or, preferably, in graphical form asshown in FIGS. 2a and 2b. Further, the monitor and processor includes acontrolled pneumatic system which controls inflation and deflation ofthe cuff 10 during which time measurements leading to the determinationof the patient's arterial volume and compliance are taken.

FIGS. 2a and 2b illustrate several preferred techniques for displayingthe information obtained through these measurements. In the upperportion of the display of FIG. 2a is a graphical display of arterialvolume (or arterial cross-sectional area, or arterial radius) versustransmural pressure. As the arteries in the peripheral body part areinfused with blood, the arteries expand and their volume increases asshown by the righthand portion of curve 20. The height of the righthandportion of the curve 20 also represents the effective radius of thearterial vessels R when the vessels are filled with blood. The slope ofthe curve 20, dV/dP, represents arterial compliance and the point atwhich dV/dP exhibits a maximum value is generally referred to as peakarterial compliance. Thus, the upper graph of FIG. 2a provides thephysician with information as to arterial volume, compliance andeffective arterial radius in the limb where the cuff is affixed.

Below the volume versus pressure graph is a graphical representation ofchanges in the effective arterial radius over time. This parameter maybe monitored by the anesthesiologist to provide information as to bodilyresponses during surgery. The illustrative curve 22 of R versus timeshown in the drawing is seen to be substantially flat, except at timeindicated by 23. This decrease in the R value may correlate for instancewith the time at which some physical intervention such as intubation orincision is performed on the patient. If the patient is not fullyanesthetized at that time, the cardiovascular system will react bycontracting the arteries of the body, and the effective radius ofarterial vessels will decline. Thus, the decline in curve 22 at point Rwould indicate to the anesthesiologist that the patient is not fullyanesthetized, and further anesthetic may be required for patient comfortand safety.

FIG. 2b shows a further display of the arterial volume and complianceinformation which would be of assistance to an anesthesiologist. In thisdisplay volume versus pressure information is displayed before theadministration of anesthesia. This curve of the patient's normalarterial volume is labelled as V(P) and the initial curve determined bythe monitor and processor. As administration of the anesthetic proceeds,the patient's cardiovascular system will respond by contracting ordilating the arterial vessels. A current volume versus pressure curve iscalculated periodically and displayed in correspondence with the initialcurve. The current curve is labelled V(P) curr. in FIG. 2b. Thus, thedisplay of FIG. 2b provides the anesthesiologist with a continuouscomparison of current arterial volume and compliance versus thepatient's normal arterial volume and compliance prior to theadministration of anesthetic.

FIGS. 3a and 3b are cross-sectional illustrations of arteries showingthe parameters measured by the monitor and processor 14. The R value isthe radius of an artery 30 as shown in FIG. 3a. Since the cuff enclosesall arterial vessels in the portion of the limb about which it iswrapped, it will be understood that the R value is not the radius of aparticular artery, but is in effect the sum of the radii of all of thearteries inside the cuff 10. Thus, the instrument provides an R valuewhich is the effective radius taken over all arterial vessels inside thecuff.

The artery 30 is defined by the arterial wall 32. The arterial wall iscomposed principally of two substances, collagen and smooth muscletissue. Collagen provides the artery with flexibility, the ability tostretch and deform. This rubber-like characteristic is one contributorto arterial compliance, and is a passive characteristic of arteries. Themuscle tissue is controlled by nerves to provide stretching anddeformation of the artery under control of the body's nervous system.This stretching and deformation is an active characteristic of theartery which also is a factor in arterial compliance.

Arterial volume and compliance are also affected in the case ofarteriosclerosis or hardening of the arteries by the buildup of fattysubstances on the inner walls of the arteries. This condition is shownin FIG. 3b, where a buildup of substances is indicated at 34 lining thewall of the artery. The ability of the artery to expand or contractunder the influence of arterial muscular contraction or blood pressurechanges is adversely affected by this lining of fatty substances, whichcan retard such motion. Since the substances also occupy a portion ofthe inner volume of the artery, the effective radius of the vessel R' isdecreased by the presence of these substances.

It may be appreciated that if the R value for an artery or a group ofarteries is known, a calculation of the cross-sectional area of theartery at that location can be performed by executing the equationA=πR². From this calculation of arterial area, arterial volume V may becalculated by multiplying the area by P, the effective length of thecuff 10 which encloses the vessels of effective area A. Thus, ameasurement of V will yield a value for R, and vice versa.

FIG. 4 illustrates an arrangement for taking measurements of arterialvolume and compliance. Shown in FIG. 4 are a limb of the body 40 incross-section, about which a blood pressure cuff 10 is wrapped. The skinline of the limb is indicated at 41. The cross-sectional view of thelimb shows the bone 42 at the center of the limb, and an artery 44passing through the limb. The artery 44 is shown expanded during thepumping of blood, before the cuff is applied and inflated. Afterinflation of the cuff to a maximal pressure, the artery will beoccluded, as shown at 44'.

The cuff 10 is connected by pneumatic tubing to a pump 50. The pump 50pumps up the cuff 10 at the start of the measurement cycle. Thearrangement of FIG. 4 is modified to perform the process of the"Hartsafe Product Concept" discussed above by the inclusion of acalibration chamber 54, which is connected to the pneumatic system. Asexplained above, at the beginning of the inflation cycle the pump 50 isstopped and one m1 of air is injected into the pneumatic system of thecuff. This may be accomplished by moving piston 56 in the chamber 54 tothe right to displace one ml of air from the chamber. Given that allelements of the pneumatic system are substantially non-compliant, thisone ml volume of air will compress the limb 40 by one m1. If all tissueand structure within the limb are assumed to be substantially liquid innature and hence substantially non-compliant, the effect of the pistondisplacement will be to displace one m1 of blood from the vascularsystem within the confines of the cuff. By taking pressure measurementsbefore and after this injection of air, the process of the Raines methodor the "Hartsafe Product Concept" calculates its calibration factor atthe outset of the measurement cycle. The pump then inflates the cuff tofully occlude the arterial vessels as shown at 44', and the deflatecycle commences. During deflation, a deflation valve opens and closes toincrementally bleed air from the pneumatic system. Measurements taken bya pressure transducer P_(T) at each pressure step are stored incorrespondence with cuff pressure level and are subsequently used in asignal conditioning (processing) step at the end of the deflation cycle.

The arrangement of FIG. 4 is seen to exhibit pneumatic structural,control, and operational complexity due to the inclusion of thecalibration chamber 54. Furthermore, the calibration step is performedonly once, at the outset of the inflation cycle. FIG. 5 illustrates aperipheral arterial volume and compliance measurement system of thepresent invention which obviates the need for such structural andoperational complexity. In FIG. 5, the blood pressure cuff 10 is wrappedaround the thigh 60 of the patient, shown in cross-section. The femur 62is shown in the center of the thigh, and the skin line of the thigh isindicated at 61. The femoral artery is illustrated at 64 in anunoccluded condition, and in an occluded condition at 64'. The cuff 10is connected by pneumatic tubing 12 to a pump 50, a pressure transducerP_(T), and a deflate valve 52. An orifice 66 of predeterminedcross-sectional area is located in the deflate valve outlet.

In operation, the pneumatic system of FIG. 5 is operated in theconventional manner of a step-deflate automated blood pressure monitorsuch as the Critikon Dinamapc 8100. The cuff 10 is inflated by the pump50 to a pressure which is in excess of systolic pressure, sufficient tofully occlude the artery 64'. The cuff pressure is stepped down, and thecuff pressures and oscillation pulses are recorded from the pressuretransducer. Two of the pressure steps during the deflate cycle are shownin FIG. 6. The cuff pressures of the two steps are P₁ and P₂, and theoscillation pulses are shown as P_(osc). The cuff pressure is steppeddown in decrements of approximately 8 mm Hg. Since the air removed fromthe pneumatic system is expelled through an orifice of known size, thevolume of air removed between each step can be calculated from a flowequation derived from the gas law PV=nRT, where P is pressure, V isvolume, n is Avogadro's constant, R is the gas constant, and T isabsolute temperature. Since the pressure on the outlet side of theorifice is ambient atmospheric pressure and the pressure on the deflatevalve side of the orifice is the cuff pressure when the deflate valve isopen, as measured by the pressure transducer relative to ambientpressure, the gas flow can be calculated from knowledge of the orificesize and the time during which the deflate valve is open. The timeduring which the deflate valve is open is shown in FIG. 6 as Δt.

In a constructed embodiment of the present invention the flow of airfrom the pneumatic system is calculated from the equation

    FLOW= (760+P)/760!· (e.sup.γln(760/(760+P)))-(e.sup.1.71 ln(760/(760+P)))!.sup.0.5

where P is the pressure across the orifice, the number 760 is anadjustment factor for nominal barometric pressure, and γ is an adiabaticconstant, typically δ=1.4. The flow through a 1 cm² orifice as afunction of the pressure across the orifice during a typical deflatecycle is represented graphically in FIG. 9. Other known methods formeasuring the flow of a fluid may also be employed; for instance, if theorifice in a given embodiment does not conform to theoretical models, itmay be approximated empirically.

Once the FLOW has been found between each step the volume of air removedduring each decrement, ΔV_(n), is computed from the equation

    ΔV.sub.n =A.sub.eq ·FLOW.sub.n ·Δt.sub.n

where A_(eq) is the equivalent area of the orifice, FLOW_(n) is the flowrate between two pressure steps, and Δt_(n) is the time during which thedeflate valve was open between the two pressure steps. The FLOW is knownfrom the preceding equation, the equivalent area of the orifice isknown, and the time during which the deflate valve is open is measuredby a digital clock which runs during the time that the valve is open.Since the FLOW calculation is done for each deflation step based uponthe known orifice and the then extant pressure, no recalibration ormodification is necessary or required for the calculated values.

From the foregoing data a ratio can be formed of the ΔV_(n) values andthe respective cuff pressure differentials at which they were obtained.The ratio is of the form

    ΔV.sub.n /ΔP(.sub.decr n)

where ΔP_(decr) is equal to P₁ -P₂ for the respective pressure step.From this ratio and the recorded values of P_(osc) the volumeoscillations can be calculated from the expression

    V.sub.osc n =P.sub.osc n ·ΔV.sub.n /ΔP.sub.decr

for each step decrement. The value of P_(osc) n used for each stepdecrement may be the amplitude of the oscillation pulses on the P₁ step,the P₂ step, or an average of the two, due to the very small variationin oscillation pulse amplitude from one step to the next. Whicheverapproach is used, it is consistently applied for the full range of stepvalues. Curves representing V_(osc) and the oscillation pulses as afunction of cuff pressure are illustrated in FIG. 10.

Using these volume oscillation values for the deflate cycle the arterialvolume curve can now be computed in a two-step procedure. The first stepis to compute a curve referred to herein as a reconstruction curve fromknowledge of the V_(osc) n values and the values of systolic anddiastolic blood pressure determined by the Dinamap™ in the conventionalmanner. The arterial volume curve is then computed by coordinate systemtransformation, by which the reconstruction curve, referenced to cuffpressure, is converted to arterial transmural pressure with reference tosystolic pressure. The equation for computing the reconstruction curveis of the form

    Recon.sub.n (P.sub.cuff)=V.sub.osc n (P.sub.cuff)+Recon.sub.n (P.sub.cuff +S-D)

where S is systolic pressure and D is diastolic pressure and thedifference of systolic minus diastolic pressure is referred to herein aspulse pressure. It is known that

    Recon.sub.n (P.sub.cuff +S-D)=0

when (P_(cuff) +S-D) is greater than P_(cuff) max, where P_(cuff) max isthe maximum cuff pressure used in a particular measurement. This followsfrom the knowledge that at maximum cuff pressure the arteries in thelimb are completely occluded. The reconstruction curve equation is seento contain the value Recon_(n) on both sides of the equation. Hence, theequation is solved recursively for n=1 . . . N where 1 . . . N are thedeflation step levels. A graphical plot of the points Recon_(n)(P_(cuff)) as a function of cuff pressure is shown by the dashed curveRecon in FIG. 11 in comparison with the V_(osc) curve previously shownin FIG. 10. It is seen that the plot of Recon converges with the V_(osc)curve above and in the vicinity of systolic pressure.

Using the Recon_(n) data points, the arterial volume may be calculatedas a function of transmural pressure by, in effect, transforming theRecon curve about the axis of systolic pressure. The equation forperforming this transformation is of the form

    P.sub.transmural =systolic pressure-P.sub.cuff

The arterial volume curve produced by this transformation is of thegeneral shape of curve 20 of FIG. 2a and the curves of FIG. 2b.

From the data points used to plot and display the arterial volume curve,the display of FIG. 2a is readily developed. A point of reference forselection of R and dV/dP may be chosen in a number of ways. The monitormay compute mean arterial pressure in the conventional manner, and usethe value of mean arterial pressure as the pressure for which R anddV/dP are chosen and displayed. Alternatively, the pressure at whichdV/dP is at a maximum, peak arterial compliance, can be used as thepressure reference for selecting R and dV/dP. As a third alternative,the physician selects a transmural pressure value on the abscissa of theupper curve of FIG. 2a as the pressure for R and dV/dP. The slope of thecurve at the selected pressure point can be calculated to determinearterial compliance dV/dP, and the amplitude of the volume curve at theselected pressure provides the R value.

During a surgical procedure the instrument is repeatedly actuatedautomatically and an R value is found each time. The R value is thendisplayed as a function of time as shown at the bottom of FIG. 2a.Alternatively, the volume curve calculated at the beginning of asurgical procedure is stored and continuously displayed with the mostrecently calculated curve in the format shown in FIG. 2b.

Another display which can be obtained from this data which would be ofuse to a clinician is a plot of dV/dP versus time, showing historicchanges in the patient's arterial compliance during a surgicalprocedure. To gauge the effectiveness of a patient's cardiovascularsystem, another alternative is to display R (or arterial area or volume)as a function of limb size. Limb size is obtained by measuring thecircumference of the limb where the cuff is attached, and entering thisinformation into the monitor and processor 14. The ratio of this R (orarterial area or volume) to circumference (or calculated limb radius orcross-sectional area) provides an indication of cardiovascularefficiency.

Alternative to the orifice of FIG. 5, a flowmeter which measures theflow of expelled air could be used to provide a direct measurement offlow volume at the output of the deflate valve 52. Another alternativeembodiment is to use a transfer volume of known capacity as shown inFIGS. 7 and 8. The transfer volume comprises all of the volumetric spacebetween an intermediate dump valve 52a and the deflate valve 52. Thesize of the vessel indicated at 58 is chosen to provide the desiredvolume of the entire transfer volume. To deflate the cuff 10, thedeflate valve 52 is closed after previous closure of the dump valve 52a.The air in the transfer volume between the two valves is now atatmospheric pressure. The dump valve 52a is then opened, and thetransfer volume becomes pressurized to the cuff pressure, which declinesto P_(tr) by reason of the expansion of pressurized air into thetransfer volume. From a knowledge of the previous cuff pressure P₁ andthe new cuff pressure P_(tr) as measured by the pressure transducer andthe known volume of the transfer volume, V_(tr), the volume ofpressurized air which has been transferred into the transfer volume andremoved from the cuff can be readily computed using the gas law

    ΔV.sub.tr =ΔV.sub.c =V.sub.tr  1-(760/(760+P.sub.tr)).sup.1/γ !

where ΔV_(c) is the volume of air removed from the cuff at pressureP_(tr) and P_(tr) is in mm Hg. This volume transferred bears arelationship to the pressure decrement which is

    ΔV.sub.tr /(P.sub.1 -P.sub.tr)

which establishes a factor from which to compute the volume oscillationon a per decrement basis:

    V.sub.osc n = ΔV.sub.tr /(P.sub.1 -P.sub.tr)!.sub.n ·P.sub.osc n

The deflate valve 52 is then opened so that both valves are in the opencondition. Air is expelled from the pneumatic system of the cuff and thepressure transducer is monitored until the pressure reaches the levelP₂, at which point the dump valve 52a is closed. The deflate valve 52 isthen closed, stabilizing the transfer volume at atmospheric pressure inpreparation for the next step decrement. The transfer volume techniqueis advantageously employed to enable use of a total pressure step P₁ -P₂which is conventional for a standard blood pressure monitor such as theDinamap™ 8100, which uses pressure step decrements of approximately 8 mmHg. Thus, arterial volume and compliance are obtained during the courseof a normal blood pressure measurement taken by a standard automatednoninvasive blood pressure monitor.

An improved reconstruction method is now described which has improvednoise rejection characteristics and, most importantly, does not requireprior knowledge of systolic and diastolic pressure. The starting pointis volume oscillations versus cuff pressure as previously described andshown in example form in FIGS. 12 and 13. It has been found that a verysimple equation relates to all of the applicable parameters whichdetermine the desired vital signs. This is shown below as Equation 1:

    VARTEST(P.sub.art)=Ao* 1+(2/π)*Arctan(P.sub.art /C) !+(Slope*P.sub.art)+E(1)

where

VARTEST(P_(art))=Estimated arterial volume as a function of arterialwall pressure

P_(art) =Pressure across the wall=P_(bp) -P_(cuff)

P_(bp) =Unknown blood pressure coefficient

Ao=Unknown area or volume coefficient

C=Unknown curve shape coefficient

Slope=Unknown coefficient for P_(art) ≧0

Slope=0 for P_(art) <0

E=Unknown coefficient for adjustment of occlusial constraint

This equation is an arctangent function with a linear term added forpositive arterial pressures. Equation 1 now has four unknown constants;Ao, SLOPE, C, and E. Ao is the arterial volume (or area) at zerotransmural pressure. SLOPE is the change in arterial volume with respectto the change in pressure across the wall. In other words, SLOPEestimates arterial compliance at high wall pressures. C is a parameterwhich describes the shape of the compliance curve, and the elasticity ofthe artery as it is being occluded. Finally, E is a parameter whichadjusts the overall curve (up or down along the volume axis) to achievezero volume at large negative pressures, thus achieving the condition ofan occluded artery at high cuff pressures. The constraint that thevolume cannot be negative is imposed.

An interesting feature of this equation is that it is monotonicallyincreasing, which is a known feature of the arterial pressure-volumecurve. In summary, this curve uses parameters which have physiologicalsignificance. It is important to note that other general curve equationscould be used, but the parameters would probably not have the directphysiological significance and, in some cases, a monitonicallyincreasing constraint would need to be imposed.

Referring to this new reconstruction method, the starting point is thevolume oscillations (Vosc), versus cuff pressures as described above andshown in example form in FIG. 12. The general mathematical form for thepressure-volume curve is assumed and plotted for an arbitrary initialchoice of parameters. Given these assumed choices out of a virtualinfinity of possible choices, one can simulate mathematically what thevolume oscillometric envelope would look like using the same cuffpressures that occurred during the actual determination sequence. Theresult of this simulation is plotted as Voscsim in FIG. 12.

The essence of this simulation is that for assumed values of Ao, C, E,SLOPE, and blood pressure, one can estimate the volume oscillationswhich would be observed by the cuff at different cuff pressures. Theestimated values can then be compared with the actual measured values.

The best way to conceptualize this result is to assume a cuff pressureof Pc and that the blood pressure is instantaneously at Pd, or diastolicpressure. Wall pressure is now at (Pd-Pc), using the sign conventionshown in FIG. 12. Under these conditions, the assumed volume of theartery is determined by Equation 1. The blood pressure now goes from Pdto Ps, or systolic pressure. Wall pressure is now (Ps-Pc) and theassumed arterial volume increases to VARTEST (Pd-Pc). Change in volumeis now VOSCSIM=VARTEST(Ps-Pcs)-VARTEST(Pd-Pcd), where Pcs and Pcd arecuff pressures at systolic and diastolic pressures, respectfully.VOSCSIM is calculated for each of the cuff pressure points whichoccurred in the actual determination process and results in the VOSCSIMvariable plotted in FIG. 12.

The correct choice of unknown parameters is the one which minimizes theerror between the measured volume oscillometric waveform and theestimated oscillometric waveform. Individual point by point errors arecalculated and then the sum of the errors squared is calculated. FIG. 13shows the results of this search optimization process.

There are two additional details. For positive values of E, which hasthe effect of sliding the assumed pressure-volume curve along the volumeaxis, there is no effect on the goodness of fit. By introducing anonlinear constant (volume cannot be negative), negative E valuesimprove the curve fit by sliding the pressure-volume curve along thevolume axis. This is conceptually consistent with knowing that theartery is occluded at cuff pressure significantly above systolicpressure..In these examples, the sum of the errors squared between theassumed pressure-volume curve and the measured volume oscillometriccurve is calculated for negative wall pressures. This is summed on aweighted basis with the original objective function and forms a newobjective function.

The second detail involves the interaction of the calibration methodwith the pressure-volume curve parameters. When air is expelled from thecuff, during a cuff decrement/calibration step, in other words for apoint C, to a point C₂, the unknown pressure-volume curve must also beaccounted for in the estimate of expelled air. This is reproduced asEquation 2 below:

EQUATION 2

    ΔV.sub.outcorr =ΔV.sub.out (from P.sub.c1 to P.sub.c2)-ΔV.sub.art (from P.sub.cl to P.sub.c2)    (2)

where

ΔV_(outcorr) =Corrected volume change

ΔV_(out) (from P_(c1) to P_(c2))=Determined from measurements

ΔV_(art) (from P_(c1) to P_(c2))=V_(art) (P_(bp-P) _(c2))-V_(art)(P_(bp) -P_(c1))

For instance, assume that the blood pressure is constant at somepressure X. At cuff pressures significantly above X, the artery iscompletely collapsed. When air is expelled, the artery volume changesonly minutely because of its complete collapse. Under these conditions,the estimate of (ΔVout) used in Equation 2 is appropriate because thearterial volume is not changing significantly during the decrement. Thesituation is different especially when wall pressure is around zero. Nowthe artery increases in volume with each decrement. The appropriateestimate of (ΔVout) is now (ΔVoutcorr) shown in Equation 2, which holdsacross all cuff pressures. The overall effect is to modify the ΔVoutestimate of Equation 2. This, in turn modifies Voscsim. FIG. 14 comparesthe two versions of Voscsim, and FIG. 15 shows optimization of thecorrected Voscsim to the measured Vosc.

Based on the disclosure of this general method which does not have priorknowledge of any of the parameters, it is clear the method can also workif one has prior knowledge of one or both of the blood pressureestimates achieved through separate means.

Systolic and pulse pressures are unknown at this point and they can bealbragetically labeled as unknowns S and P. The problem now involves sixor five unknown constants, and the inverse problem is a choice ofconstants which will best match the volume pressure envelope Vosc, asshown in FIG. 12. Such estimated values A_(O), C, E and slope produce aguessed pressure-volume curve as shown as Voscsim in FIG. 12.

One solution to this problem is to assume diastolic and systolicpressure values, these are initially chosen at 120 mm Hg and 40 mm Hg,because they are the normal values of the whole population. Given theseassumed choices one out of a virtual infinity of possible choices, onecan simulate mathematically what the volume oscillometric envelope wouldlook like using the same cuff pressures that occur during an actualdetermination sequence.

As seen in FIG. 14, there is described an improved sequence forresolving the equation. This method now determines the parameters usinginitial assumed values of A_(O), C, E, Slope, S and P. This methodtherefore, estimates the volume change observed at various cuffpressures. The best way to conceptualize this theory is to assume a cuffpressure of Pc, so that blood pressure is instantaneously shown as S.Wall pressure at this given Pc is now determined as S-Pc. Under theseconditions, the assumed volume of the artery is known and can be plottedagainst wall pressure as in FIG. 14. The blood pressure now varies fromS to S-P. Wall pressure also varies from (S-P) to (S-P-Pc), and theassumed arterial volume decreases accordingly.

Thus, an assumed shape of the pressure curved affects both the dV/Pestimate obtained during the calibration decrement, as well as the shapein which the simulated volume oscillometric curve takes. A best choiceof parameters is still the set which minimizes the error between thevolume of the curved obtained from this determination. What thesedeterminations result in are actual measurements of all the parameters,including diastolic and systolic pressures, as compared to previousmethods which only determine parameters on a scaled basis. In otherwords, there is no scaling of any of the parameters values, and actualnumbers can be obtained. This will allow the user to obtain much morerobust meanings for these readings and, therefore, allows for immediateand applicable in using these known methods.

It is, therefore, meant that the following claims and their equivalentscover the scope of the invention as previously described.

What is claimed is:
 1. Instrumentation for performing measurements ofthe vital signs comprising:an inflatable cuff; means, connected to saidcuff for inflating and deflating said cuff; means for detecting pressurelevels of said cuff; means for removing quantified decrements of airfrom said cuff; means for computing values of oscillation volume fromsaid quantified decrements of air; and means for computing arterialvolume as a function of said values of said oscillation volume and cuffpressure, wherein said computing means comprise means for estimatingsaid arterial volume and creating a volume versus pressure curve. 2.Instrumentation according to claim 1, wherein said computing meanscomputes arterial volume as a function of said values of oscillationvolume and cuff pressure.
 3. Instrumentation according to claim 1,wherein said computing means computes arterial volume as a function oftransmural pressure.
 4. Instrumentation according to claim 1 whereinsaid computing means computes systolic and diastolic pressure as afunction of arterial volume.
 5. Instrumentation for performingmeasurements of vital signs comprising:an inflatable cuff; means forinflating said cuff; means for detecting pressure levels within saidcuff; means for expelling air from said cuff; and computing means forcomputing arterial volume as a function of said air expelled from saidcuff and said detected pressure levels, wherein said computing means isalso for computing systolic and pulse pressures using said computedarterial volume.
 6. Instrumentation according to claim 5, wherein saidexpelling means comprises an orifice of known flow characteristics. 7.Instrumentation according to claim 5, wherein said expelling meanscomprises a volume of known characteristics which is selectively coupledto said cuff.
 8. Instrumentation according to claim 7, wherein said airexpelling means includes a deflate valve for expelling air from saidcuff through said orifice; and wherein said computing meansincludes:means for computing flow through said orifice as a function ofcuff pressure; means for computing the volume flow of air expelled as afunction of flow, orifice characteristics, and the time said deflatevalve is open; and means for computing arterial volume as a function ofsaid volume of air expelled and cuff pressure.
 9. Instrumentation forperforming measurements of vital signs comprising:an inflatable cuff;means for inflating said cuff; means for detecting pressure levelswithin said cuff; means for expelling air from said cuff; and computingmeans for computing arterial volume as a function of said air expelledfrom said cuff and said detected pressure levels, wherein said computingmeans is also capable of creating a volume versus pressure curve. 10.The instrumentation of claim 9 wherein said computing means is also forcomputing systolic and pulse pressures using said computed arterialvolume.
 11. Instrumentation for performing measurements of arterialvolume comprising:an inflatable cuff; means, connected to said cuff, forinflating and deflating said cuff; means for detecting pressure levelswithin said cuff; means for computing arterial volume as a function ofsaid detected pressure levels within said cuff; means for computingarterial capacity as a function of pressure; and further including meansfor computing systolic and pulse pressures as a function of saidarterial volume and computed pressure levels.
 12. Instrumentationaccording to claim 11, including displaying means which displayssystolic and pulse pressures.